Russia and Ukraine stepped up security on the de facto perimeter between Crimea and mainland Ukraine on Thursday, a day after Moscow accused Kiev of planning terror attacks on the peninsula.

The alleged incidents, which have raised tensions over Crimea to the highest level since Russia annexed the region from Ukraine in 2014, took place over the weekend. Russias FSB security service claims to have detained a Ukrainian terror cell with 20 homemade explosive devices inside Crimea, and repulsed fire from across the border in a separate incident. The FSB said one of its officers and a Russian soldier died during the clashes.

The FSB statement on Wednesday was swiftly followed by tough statements from Russias president, Vladimir Putin, who accused Ukraine of terrorism. We patently will not let such things slide by, Putin said.

On Thursday, Putin satisfied his security council, and the group discussed additional measures for ensuring security for citizens and essential infrastructure in Crimea, according to a Kremlin statement. The statement added: Scenarios were carefully considered for anti-terrorist security measures at the land perimeter, in the water and within the national airspace of Crimea.

The international community responded with alarm to the increased tension on the peninsula, and European and US envoys carried scepticism over the Russian claims.

The US ambassador to Ukraine, Geoffrey Pyatt, wrote on Twitter: Russia has a record of often levying false accusations at Ukraine to deflect attention from its own illegal actions.

Russia seized Crimea in 2014, after the Maidan revolution in Kiev led to the countrys then chairperson, Viktor Yanukovych, fleeing. The annexation, carried out by Russian soldiers wearing unmarked uniforms, led to western sanctions against Russia. Further sanctions were later introduced over Russian is supportive of rebels in east Ukraine.

Russia proved no sign of toning down the rhetoric on Thursday, and the Kremlin-appointed head of Crimea, Sergei Aksyonov, suggested executing those who had been arrested. With saboteur I think we should act as farmers do with crows who spoil the crops. Kill them and hang their bodies at the border to put off others, and build them understand that Russia will not permit anyone to risk the well-being of peaceful citizens or servicemen in Crimea, Aksyonov told Russian nation television. He also accused the US state department of being behind the alleged attacks.

Russian bureaux quoted unnamed security sources suggesting that those in detention had confessed to a plan to blow up infrastructure in Crimea , not to cause loss of life but to destroy the tourist season.

Poroshenko said it was Russia who was using methods of terror inside Ukraine, with its support for the rebel militia forces in the eastern part of the country. Ukraine resolutely condemns terrorism in all its sorts and shapes. We would never ever use terror to de-occupy Crimea, the Ukrainian chairman said in a statement. Russia provides money and arms to support terrorism in Ukraine. It became a state-led policy on the occupied areas of Crimea and eastern Ukraine that resembles the Soviet Great Terror.